Differences in the frequency of pharmacogenomic variants may influence inter-population variability in drug efficacy and risk of adverse drug reactions (ADRs). We investigated the diversity of ∼ 4500 genetic variants in key drug-biotransformation and -response genes among three South East Asian populations compared with individuals of European ancestry. We compared rates of reported ADRs in these Asian populations to determine if the allelic differentiation corresponded to an excess of the associated ADR. We identified an excess of ADRs related to clopidogrel in Singaporean Chinese, consistent with a higher frequency of a known risk variant in CYP2C19 in that population. We also observed an excess of ADRs related to platinum compounds in Singaporean CHS, despite a very low frequency of known ADR risk variants, suggesting the presence of additional genetic and non-genetic risk factors. Our results point to substantial diversity at specific pharmacogenomic loci that may contribute to inter-population variability in drug response phenotypes.